1. Field of the Invention
The present invention relates generally to devices used to measure DC currents, and, more specifically, it relates to an improved non-intrusive DC current monitor with a substantially improved accuracy, sampling rate and reduction in noise.
2. Brief Description of the Prior Art
It is often necessary to measure the flow of electrical current within an electronic system. The most common method of measurement is to pass a current through a resistor and measure the resulting voltage drop thereacross, the voltage drop being representative of the passing current. As is generally known, the resistor used in making the measurement adds to the loading on the circuit through which the DC current must pass. This increased load can cause a substantial power loss, especially in high power systems where there is significant DC current flow.
Alternative current measuring devices are available which permit potential free measurement of direct current. An example of this type of device is the Hall-effect sensor which is a non-intrusive current converter. A Hall-type of device senses the magnetic field of a current and converts it into electrical signals in proportion to field intensity. The field intensity, is representative of the magnitude of the current being measured. However, these types of devices suffer from fundamental limitations in their stability and accuracy which may make them ineffective for use in precision systems. For example, these current converters have a zero-offset current which exhibits a characteristic drift behavior. This is caused by factors such as aging or temperature changes. Although this zero drift is small, significant distortions in measurements often result.
Another type of DC current measuring device, an example of which is disclosed in U.S. Pat. No. 4,682,100, makes use of a transformer-like device to measure the current in a non-intrusive fashion. The DC current to be measured is supplied through a primary winding, which causes a magnetic flux in the transformer core. This flux is compensated by a current supplied to a secondary winding until the flux in the core is equalized at a zero net-flux state. This compensating current through the secondary winding constitutes a copy of the current in the primary winding transformed over the transforming ratio of the transformer. Devices using similar technology are disclosed in U.S. Pat. No. 3,368,011, U.S. Pat. No. 4,454,553, and in the publication Powercon II, page I-1, April, 1984.
None of those prior art devices, however, are capable of providing reliable and accurate data at low values of bus current, thus limiting their range of effectiveness. Additionally, many of these devices induce noise into the bus when using compensating current to make a bus current measurement. Another limitiation is that none of those prior art devices provide an extension to measurement of bilateral current flow.